As one of methods for forming a film on a substrate, there is known an atomic layer deposition (ALD) method. In the ALD method, as a precursor gas is supplied into a chamber in which a processing target substrate is accommodated, atoms or molecules of the precursor gas containing a constitution element of a thin film are adsorbed onto the processing target substrate. Then, the atoms or molecules of the precursor gas excessively adsorbed on the processing target substrate is removed by a purge gas supplied into the chamber. Thereafter, a reactant gas containing a constituent element of the thin film is supplied into the chamber, and the processing target substrate having the atoms or molecules of the precursor gas adsorbed thereto is exposed to active species of the reactant gas, so that the required thin film is formed on the processing target substrate. The active species of the reactant gas are generated by, for example, plasma. In the ALD method, as a cycle including a single adsorption process and a single reaction process is repeated multiple times, a film of the atoms or molecules contained in the precursor gas is deposited on the substrate layer by layer.
Patent Document 1: Japanese Patent Laid-open Publication No. 2009-016814
In each process of the film formation using the ALD method, a state within the chamber such as a pressure therein is controlled based on a measurement value measured by various kinds of sensors such as a pressure gauge. In many cases, however, sensor surfaces of such sensors may be exposed to a processing space within the chamber. As a result, the same as the film is formed on the processing target substrate, a reaction product caused by the precursor gas and the reactant gas is also deposited on the sensor surfaces of the sensors which are exposed to the processing space within the chamber. Accordingly, as the cycle of the ALD is repeated, sensitivity of the sensors may be deteriorated, and accuracy of various kinds of measurement values measured by the sensors may be degraded. Thus, it may be difficult to control the state within the chamber to a required state, and accuracy, such as the quality of the film formed on the processing target substrate, may be degraded.